Rename v5 FontAwesome icons to match v6 name

While FontAwesome provides aliases for all icons, we switch to the new names for consistency. See all changes at:
https://fontawesome.com/docs/web/setup/upgrade/whats-changed#icons-renamed-in-version-6

lib/assets/javascripts/flash.js

@@ -26,7 +26,7 @@ $( document ).on('turbolinks:load', function() {
     $('.flash').each(function() {
       var container = $(this);
       var message = container.find('p');
-      var button = container.find('span.fa-times');
+      var button = container.find('span.fa-xmark');
 
       var hide = function() {
         container.slideUp(function () {

lib/assets/stylesheets/flash.css.scss

@@ -5,7 +5,7 @@
 .flash {
   display: none;
 
-  span.fa-times {
+  span.fa-xmark {
     position: relative;
     top: -1.2em;
     float: right;